1. Field of the Invention
The present invention relates to a substrate processing method and method of manufacturing a semiconductor substrate and, more particularly, to a processing method of processing a used first substrate that remains after a bonded substrate stack is formed by bonding a first substrate having a separation layer and a transfer layer on the separation layer to a second substrate, and the bonded substrate stack is separated mainly at the separation layer to transfer a partial region of the transfer layer to the second substrate, and a method of manufacturing a semiconductor substrate using the processing method.
2. Description of the Related Art
A substrate (SOI substrate) having an SOI (Silicon On Insulator) structure is known as a substrate having a single-crystal Si layer on an insulating layer. A device using this SOI substrate has many advantages that cannot be achieved by ordinary Si substrates. Examples of the advantages are as follows.
(1) The integration degree can be increased because dielectric isolation is easy.
(2) The radiation resistance can be increased.
(3) The operating speed of the device can be increased because the stray capacitance is small.
(4) No well step is necessary.
(5) Latch-up can be prevented.
(6) A complete depletion type field effect transistor can be formed by thin film formation.
Since an SOI structure has the above various advantages, researches have been made on its formation method for several decades.
As one SOI technology, the SOS (Silicon On Sapphire) technology by which Si is heteroepitaxially grown on a single-crystal sapphire substrate by CVD (Chemical Vapor Deposition) has been known for a long time. This SOS technology once earned a reputation as the most matured SOI technology. However, the SOS technology has not been put into practical use to date because, e.g., a large amount of crystal defects are produced by lattice mismatch in the interface between the Si layer and the underlying sapphire substrate, aluminum that forms the sapphire substrate mixes in the Si layer, the substrate is expensive, and it is difficult to obtain a large area.
Attempts have recently been made to realize the SOI structure without using any sapphire substrate. The attempts are roughly classified into two methods.
In the first method, the surface of a single-crystal Si substrate is oxidized, and a window is formed in the oxide film (SiO2 layer) to partially expose the Si substrate. Single-crystal Si is epitaxially grown laterally using the exposed portion as a seed, thereby forming a single-crystal Si layer on SiO2 (in this method, an Si layer is deposited on an SiO2 layer).
In the second method, a single-crystal Si substrate itself is used as an active layer, and an SiO2layer is formed on the lower surface of the substrate (in this method, no Si layer is deposited).
As a means for realizing the first method, a method of directly epitaxially growing single-crystal Si in the horizontal direction from the single-crystal Si layer by CVD (CVD), a method of depositing amorphous Si and epitaxially growing single-crystal Si laterally in the solid phase by annealing (solid phase epitaxial growth), a method of irradiating an amorphous silicon layer or a polysilicon layer with a focused energy beam such as an electron beam or laser beam to grow a single-crystal Si layer on an SiO2 layer by melting recrystallization (beam annealing), or a method of scanning band-shaped melting regions by a rod-like heater (zone melting recrystallization) is known.
All of these methods have both advantages and disadvantages and many problems of controllability, productivity, uniformity, and quality, and therefore have not been put into practical use in terms of industrial applications. For example, CVD requires sacrifice oxidation to form a flat thin film. Solid phase epitaxial growth is poor in crystallinity. In beam annealing, the process time required to scan the focused beam and controllability for beam superposition or focal point adjustment pose problems. Zone melting recrystallization is the most matured technique, and relatively large-scaled integrated circuits have been fabricated on a trial basis. However, since a number of crystal defects such as a subboundary undesirably remain, minority carrier devices cannot be created.
As the above second method, i.e., as the method without using the Si substrate as a seed for epitaxial growth, the following four techniques can be used.
As the first technique, an oxide film is formed on a single-crystal Si substrate having a V-shaped groove formed in the surface by anisotropic etching. A polysilicon layer having nearly the same thickness as that of the single-crystal Si substrate is deposited on the oxide film. After this, the single-crystal Si substrate is polished from the lower surface, thereby forming, on the thick polysilicon layer, a substrate having a single-crystal Si region surrounded and dielectrically isolated by the V-shaped groove. With this technique, a substrate having satisfactory crystallinity can be formed. However, there are problems of controllability and productivity in association with the process of depositing polysilicon as thick as several hundred micron or the process of polishing the single-crystal Si substrate from the lower surface to leave the isolated Si active layer.
The second technique is SIMOX (Separation by Ion Implanted Oxygen). In this technique, oxygen ions are implanted into a single-crystal Si substrate to form an SiO2 layer. In this technique, to form an SiO2 layer in a substrate, oxygen ions must be implanted at a dose of 1016 (ions/cm2) or more. This implantation takes a long time to result in low productivity and high manufacturing cost. In addition, since a number of crystal defects are generated, the quality is too low to manufacture minority carrier devices.
As the third technique, an SOI structure is formed by dielectric isolation by oxidizing a porous Si layer. In this technique, an n-type Si island is formed on the surface of a p-type single-crystal Si substrate by proton ion implantation (Imai et al., J. Crystal Growth, vol. 63, 547 (1983)) or epitaxial growth and patterning. This substrate is anodized in an HF solution to convert only the p-type Si substrate around the n-type Si island into a porous structure. After this, the n-type Si island is dielectrically isolated by accelerated oxidation. In this technique, since the Si region to be isolated must be determined before the device process, the degree of freedom in device design is limited.
As the fourth technique, an SOI structure is formed by bonding a single-crystal Si substrate to another thermally oxidized single-crystal Si substrate by annealing or an adhesive. In this technique, an active layer for forming a device must be uniformly thin. More specifically, a single-crystal Si substrate having a thickness of several hundred micron must be thinned down to the micron order or less.
To thin the substrate, polishing or selective etching can be used.
A single-crystal Si substrate can hardly be uniformly thinned by polishing. Especially, in thinning to the submicron order, the variation range is several ten %. As the wafer size becomes large, this difficulty becomes more pronounced.
Selective etching is effective to uniformly thin the substrate. However, the selectivity ratio is as low as about 102, the surface planarity after etching is poor, and the crystallinity of the SOI layer is unsatisfactory.
A transparent substrate represented by a glass substrate is important in forming a contact sensor as a light-receiving element or a projection liquid crystal display device. To realize highly precise pixels (picture elements) having higher density and resolution for the sensor or display device, a high-performance driving element is required. For this purpose, a demand has arisen for a technique of forming a single-crystal Si layer having excellent crystallinity on a transparent substrate.
However, when an Si layer is deposited on a transparent substrate represented by a glass substrate, only an amorphous Si layer or a polysilicon layer is obtained. This is because the transparent substrate has an amorphous crystal structure, and the Si layer formed on the substrate reflects the disorderliness of the crystal structure of the transparent substrate.
The present applicant has disclosed a new SOI technology in Japanese Patent Laid-Open No. 5-21338. In this technique, a first substrate obtained by forming a porous layer on a single-crystal Si substrate and a non-porous single-crystal layer on its surface is bonded to a second substrate via an insulating layer. After this, the bonded substrate stack is separated into two substrates at the porous layer, thereby transferring the non-porous single-crystal layer to the second substrate. This technique is advantageous because the film thickness uniformity of the SOI layer is good, the crystal defect density in the SOI layer can be decreased, the surface planarity of the SOI layer is good, no expensive manufacturing apparatus with special specifications is required, and SOI substrates having about several hundred-xc3x85 to 10-xcexcmn thick SOI films can be manufactured by a single manufacturing apparatus.
Porous Si was found in 1956 by Uhlir et al. who were studying electropolishing of semiconductors (A. Uhlir, Bell Syst. Tech. J., vol. 35, 333 (1956)). Porous Si can be formed by anodizing an Si substrate in an HF solution.
Unagami et al. studied the dissolution reaction of Si upon anodizing and reported that holes were necessary for anodizing reaction of Si in an HF solution, and the reaction was as follows (T. Unagami, J. Electrochem. Soc., vol. 127, 476 (1980)).
Si+2HF+(2xe2x88x92n)e30xe2x86x92SiF2+2H++ne31 
SiF2+2HFxe2x86x92SiF4+H2 
SiF4+2HFxe2x86x92H2SiF6 
or
Si+4HF+(4xe2x88x92xcex)e+xe2x86x92SiF4+4H++xcexe31 
SiF4+2HFxe2x86x92H2SiF6 
where e30  and e31  represent a hole and an electron, respectively, and n and xcex are the number of holes necessary to dissolve one Si atom. According to them, when n greater than 2 or xcex greater than 4, porous Si is formed.
The above fact suggests that p-type Si having holes is converted into porous Si while n-type Si is not converted. The selectivity in this conversion has been reported by Nagano et al. and Imai (Nagano, Nakajima, Anno, Onaka, and Kajiwara, IEICE Technical Report, vol. 79, SSD79-9549 (1979)), (K. Imai, Solid-State Electronics, vol. 24, 159 (1981)).
However, it has also been reported that n-type at a high concentration is converted into porous Si (R. P. Holmstrom and J. Y. Chi, Appl. Phys. Lett., vol. 42, 386 (1983)). Hence, it is important to select a substrate which can be converted into a porous Si substrate independently of p- or n-type.
To form a porous layer, in addition to anodization, ions may be implanted into a silicon substrate.
An SOI substrate is formed using a normal single-crystal Si substrate or the like as a material, and the manufacturing cost is higher than that of a normal single-crystal Si substrate.
This also applies to the method described in Japanese Patent Laid-Open No. 5-21338, i.e., the method in which a substrate (to be referred to as a bonded substrate stack hereinafter) obtained by bonding a first substrate having a non-porous layer such as a single-crystal Si layer on a porous layer to a second substrate via an insulating layer is separated at the porous layer, thereby transferring the non-porous layer formed on the first substrate side to the second substrate.
In consideration of this situation, the present applicant has also disclosed, in Japanese Patent Laid-Open No. 7-302889, a technique of bonding first and second substrates, separating the first substrate from the second substrate without breaking the first substrate, smoothing the surface of the separated first substrate, forming a porous layer again on the first substrate, and reusing this substrate. Since the first substrate is not wasted, this technique is advantageous in largely reducing the manufacturing cost and simplifying the manufacturing process.
For example, for the method disclosed in Japanese Patent Laid-Open No. 7-302889, a demand for an efficient method of planarizing the surface of the separated first substrate has arisen.
Mitsuya has reported in xe2x80x9cSilicon-on-Insulator Manufacturing Technology, H-10xe2x80x9d of xe2x80x9cSEMICON WEST 98xe2x80x9d that after separation of a substrate formed by bonding first and second substrates, a step difference of 0.3 xcexcm is present in the periphery, and to planarize the surface of the first substrate by polishing in this state, the surface of the first substrate must be polished by 1 xcexcm or more.
It is an object of the present invention to easily planarize a used first substrate that remains after a bonded substrate stack is formed by bonding a first substrate having a separation layer and a transfer layer on the separation layer to a second substrate, and the bonded substrate stack is separated mainly at the separation layer to transfer a partial region of the transfer layer to the second substrate, whereby reuse of the first substrate is facilitated.
According to the first aspect of the present invention, there is provided a substrate processing method of processing a used first substrate that remains after a bonded substrate stack is formed by bonding a first substrate having a separation layer and a transfer layer on the separation layer to a second substrate, and the bonded substrate stack is separated mainly at the separation layer to transfer a partial region of the transfer layer to the second substrate, comprising the transfer layer removal step of removing the transfer layer remaining on the used first substrate, and the separation layer removal step of removing the separation layer remaining on a surface of the used first substrate.
In the processing method according to the first aspect of the present invention, for example, the transfer layer removal step preferably comprises selectively removing the transfer layer remaining on an upper surface of the used first substrate.
In the processing method according to the first aspect of the present invention, for example, the transfer layer remaining on the used first substrate is preferably present at least an outer peripheral portion of an upper surface of the first substrate.
In the processing method according to the first aspect of the present invention, for example, the transfer layer remaining on the used first substrate is preferably present at least an outer peripheral portion and edge portion of an upper surface of the first substrate.
In the processing method according to the first aspect of the present invention, for example, the transfer layer remaining on the used first substrate is preferably present at least an outer peripheral portion and edge portion of upper and lower surfaces of the first substrate.
In the processing method according to the first aspect of the present invention, for example, the transfer layer remaining on the used first substrate is preferably present at least an outer peripheral portion and edge portion of an upper surface and on a lower surface of the first substrate.
In the processing method according to the first aspect of the present invention, preferably, for example, the transfer layer sequentially has a first layer and a second layer on the separation layer, and the transfer layer removal step comprises the first step of removing the second layer remaining on the used first substrate, and the second step of removing the first layer remaining on the used first substrate.
In the processing method according to the first aspect of the present invention, for example, the transfer layer preferably includes a semiconductor layer.
In the processing method according to the first aspect of the present invention, for example, the transfer layer preferably includes an Si layer.
In the processing method according to the first aspect of the present invention, for example, the transfer layer preferably includes a single-crystal Si layer.
In the processing method according to the first aspect of the present invention, preferably, for example, the first layer is a single-crystal Si layer, and the second layer is an SiO2 layer.
In the processing method according to the first aspect of the present invention, for example, the transfer layer preferably includes at least one of a Ge layer, an SiGe layer, an SiC layer, and a C layer.
In the processing method according to the first aspect of the present invention, for example, the transfer layer preferably includes a compound semiconductor layer.
In the processing method according to the first aspect of the present invention, for example, the separation layer is preferably a porous layer.
In the processing method according to the first aspect of the present invention, for example, the first substrate preferably has, as the separation layer, a porous layer obtained by anodizing a surface of a single-crystal Si substrate, and the transfer layer on the porous layer.
In the processing method according to the first aspect of the present invention, for example, the first substrate preferably has, as the separation layer, a porous layer obtained by implanting ions into a single-crystal Si substrate, and the transfer layer on the porous layer.
In the processing method according to the first aspect of the present invention, for example, the first substrate is preferably prepared by forming the separation layer and the transfer layer on an Si substrate.
In the processing method according to the first aspect of the present invention, for example, the second substrate is preferably one of an Si substrate, an Si substrate having an oxide film, a transparent substrate, and an insulating substrate.
In the processing method according to the first aspect of the present invention, for example, the transfer layer removal step preferably comprises etching the transfer layer remaining on the used first substrate using a solution containing hydrofluoric acid.
In the processing method according to the first aspect of the present invention, for example, the first step preferably comprises etching the single-crystal Si layer as the first layer using one of hydrofluoric acid and buffered hydrofluoric acid.
In the processing method according to the first aspect of the present invention, for example, the second step preferably comprises etching the SiO2 layer as the second layer using a solution mixture of hydrofluoric acid, acetic acid, and nitric acid.
In the processing method according to the first aspect of the present invention, for example, the transfer layer removal step preferably comprises etching the transfer layer remaining on the used first substrate by dry etching.
In the processing method according to the first aspect of the present invention, for example, the separation layer removal step preferably comprises selectively removing the separation layer.
In the processing method according to the first aspect of the present invention, for example, the separation layer removal step preferably comprises selectively removing the separation layer by wet etching.
In the processing method according to the first aspect of the present invention, for example, as an etchant, one material selected from the group consisting of 1) hydrofluoric acid, 2) a solution mixture prepared by adding at least one of an alcohol and hydrogen peroxide to hydrofluoric acid, 3) buffered hydrofluoric acid, and 4) a solution mixture prepared by adding at least one of an alcohol and hydrogen peroxide to buffered hydrofluoric acid is preferably used.
In the processing method according to the first aspect of the present invention, for example, the separation layer removal step preferably comprises selectively removing the separation layer by polishing.
The processing method according to the first aspect of the present invention preferably further comprises, e.g., the planarization step of planarizing an upper surface of the used first substrate after the separation layer removal step.
In the processing method according to the first aspect of the present invention, for example, the planarization step preferably comprises the step of annealing the upper surface of the used first substrate in an atmosphere containing hydrogen.
In the processing method according to the first aspect of the present invention, for example, the planarization step preferably comprises the step of planarizing the upper surface of the used first substrate by polishing.
According to the second aspect of the present invention, there is provided a substrate processing method of processing a used substrate that remains after a transfer layer of a substrate having a separation layer and the transfer layer on the separation layer is transferred to another object, comprising the transfer layer removal step of removing the transfer layer remaining on the used substrate, and the separation layer removal step of removing the separation layer remaining on a surface of the used substrate.
According to the third aspect of the present invention, there is provided a substrate processing method of removing a porous layer and other layers from a substrate having the porous layer and the other layers at an outer peripheral portion of the porous layer, comprising the steps of removing the other layers, and removing the porous layer.
According to the fourth aspect of the present invention, there is provided a method of manufacturing semiconductor substrates, comprising the preparation step of bonding a first substrate having a separation layer and, on the separation layer, a transfer layer including a semiconductor layer to an independently prepared second substrate to prepare a bonded substrate stack, the transfer step of separating the bonded substrate stack mainly at the separation layer to transfer a partial region of the transfer layer to a surface of the second substrate, thereby preparing a semiconductor substrate having the transfer layer on a surface, the transfer layer removal step of removing the transfer layer remaining on the first substrate after the transfer step, and the separation layer removal step of removing the separation layer remaining on the surface of the first substrate after the transfer layer removal step, wherein the semiconductor substrates are obtained while executing a series of processes of reusing the first substrate after the separation layer removal step as a material used to prepare a bonded substrate stack in the preparation step.
Further objects, features and advantages of the present invention will become apparent from the following detailed description of the embodiments of the present invention with reference to the accompanying drawings.